Directions: Stare into the center of any of the black boxes in the above grid. Notice that the edges of the box may appear gray. Now stare anywhere in the grid and notice how, at the intersection of each of the white lines between the boxes, black dots appear. However, when you stare at those intersections, the black dot in that area disappears, but the others remain. How can this be?
The gray that appears at the edges of the black squares when one looks at them is because of something called lateral inhibition. This is when certain light-receiving cells are activated and those around them shut down. Since these other cells shut down and do not receive light, the outer edges of the black boxes appear as if they have become gray, or a lighter form of black.
The black dots that appear in the intersections of the white lines are also because of a form of inhibition. Because when a person stares at a white line, they are surrounded on two sides by black colorating. However, when a person stares at just the intersection, it is surrounded by black on four sides. The cells are inhibited, which causes the black dots to form in the intersections.
Directions: Stare into the center of any of the black boxes in the above grid. Notice that the edges of the box may appear gray. Now stare anywhere in the grid and notice how, at the intersection of each of the white lines between the boxes, black dots appear. However, when you stare at those intersections, the black dot in that area disappears, but the others remain. How can this be?
The gray that appears at the edges of the black squares when one looks at them is because of something called lateral inhibition. This is when certain light-receiving cells are activated and those around them shut down. Since these other cells shut down and do not receive light, the outer edges of the black boxes appear as if they have become gray, or a lighter form of black.
The black dots that appear in the intersections of the white lines are also because of a form of inhibition. Because when a person stares at a white line, they are surrounded on two sides by black colorating. However, when a person stares at just the intersection, it is surrounded by black on four sides. The cells are inhibited, which causes the black dots to form in the intersections.
http://www.pbs.org/wnet/brain/illusions/color.html